This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improving data transfer operations by utilizing an automatic dummy load during a head switching operation.
Hard disc drives are used in modem computer systems to enable users to store and retrieve vast amounts of data in a fast and efficient manner. A typical disc drive is generally composed of a head/disc assembly (HDA) which houses requisite mechanical portions of the drive and a printed wiring assembly (PWA) which supports requisite electronic portions of the drive.
The HDA includes a base deck to which various components are mounted and a top cover which cooperates with the base deck to form a sealed housing to reduce particulate contamination. Within the housing, a disc stack is formed from one or more magnetic recording discs which are axially aligned for rotation by a spindle motor at a constant, high speed, such as 10,000 revolutions per minute during normal disc drive operation.
A rotary actuator assembly is mounted adjacent the disc stack and includes a plurality of rigid arms which extend into the stack between adjacent discs, as well as above and below the top and bottom discs. The rigid arms support flexible suspension assemblies, which in turn support a corresponding number of read/write heads adjacent the surfaces of the discs. One head is provided for each disc surface to read data from and to write data to the corresponding surface.
Of particular interest are magneto-resistive (MR) heads, which utilize thin-film inductive write elements to write data and MR read elements to read previously written data. A typical MR read element is formed from an alloy of materials so as to have a baseline electrical resistance which varies in the presence of a magnetic field of a selected orientation. By passing a bias current (this bias current may be generated by a means of current, voltage or power biasing method) through the MR element, the selective magnetization of a corresponding track can be determined in relation to variations in voltage detected across the MR element.
A preamplifier/driver circuit is typically mounted to the side of the actuator assembly and includes a write driver circuit to apply write currents to a selected write element during a write operation. The preamplifier/driver circuit further includes a read bias current source which is sequentially connected to the appropriate read element to effect a read operation.
The electronics provided on the disc drive PWA primarily serve to control the operation of the HDA and to communicate with a host computer in which the disc drive is mounted. Generally, the top level functional blocks on the PWA include a read/write channel which controls the reading and writing of data from and to the discs, a spindle motor control circuit which controls the rotation of the spindle motor, and a servo control circuit which controls the position of the heads.
Aspects of a typical servo control circuit are discussed in U.S. Pat. No. 5,262,907 issued to Duffy et al., assigned to the assignee of the present invention. The servo control circuit positions the heads relative to the tracks through the application of current to a coil of a voice coil motor (VCM) within the HDA, the coil being mounted to the actuator opposite the heads. The tracks are defined from servo data written to servo fields on the surfaces of the discs during the manufacturing of the HDA. User data are stored in user data fields that are formed between adjacent servo fields. The servo data are periodically transduced and provided to a servo processor which carries out positioning operations (such as seeks and track following modes) in accordance with programming and commands from a top level disc drive processor.
The preamp is used to multiplex the various read and write elements of the various heads onto a single channel to the read/write channel circuitry. To this end, a typical preamp includes a buffer that contains several digital registers that are controlled by the servo processor via a specialized serial link so that command inputs, such as the identification of the selected head, the magnitude of read bias current and the magnitude of write current can be programmably selectable. For example, to effectuate a head switch, the servo processor writes a new head value to the preamp head register to indicate the new location for the head. Since many registers must be updated at every head switch, the minimum head switch time is constrained by the time it takes to complete all the register writes.
MR heads, while providing substantially greater levels of performance over prior generation inductive heads, are also more susceptible to damage due to an overcurrent condition. While nominally identical, process variations will usually result in MR heads having different levels of maximum permissible read bias current magnitude. As it is important to not apply more than the maximum permissible read bias current to a particular head, disc drives typically carry out a head switch operation wherein the preamp switches the interconnection of the read/write channel from a first head to a second head by first switching from the first head to a dummy load, that is, a non-head resistive current path; that is, the read bias current previously passed through the first head is shunted into an internal or an external resistor. This condition is sometimes referred to as xe2x80x9cdummy mode.xe2x80x9d Setting the preamp in dummy mode requires the writing of the appropriate command bit or bits to the associated preamp command register. Once the current is directed to the dummy load, a second preamp write command is executed to switch the read bias current to the new level appropriate for the second head to be selected, after which a third write command is executed to write the address of the second head into the appropriate selected head register. Finally, a fourth write command toggles the command register to remove the preamp from dummy mode, causing the new, second read bias current magnitude to be applied to the second head. Thus, while such an approach minimizes the potential for damage due to overcurrent conditions in the heads, multiple preamp accesses are required, which requires both an undesired amount of time as well as substantial overhead from the servo processor, which is also carrying out other servo positioning operations.
Accordingly, there is a continued need for improvements in the manner in which head switching occurs to improve the operational performance of a disc drive, and it is to such improvements that the present invention is directed.
The present invention is directed to an apparatus and method for providing improved head switching in a disc drive.
In accordance with preferred embodiments, a disc drive includes a head/disc assembly (HDA) housing a plurality of disc recording surfaces and an actuator assembly which supports a corresponding plurality of heads adjacent the surfaces. A preamplifier circuit is mounted to the actuator assembly and includes a bias current source which generates a first bias current of selected magnitude to apply to a first head.
A sequence control circuit detects the input of a multi-bit head selection value indicative of a second head to a buffer of the preamp of the disc drive. The sequence control circuit simultaneously switches the first bias current from the first head to a dummy load before completion of the input of the head selection value in the buffer. After a second bias current of a second magnitude is written to the buffer, a predetermined time interval elapses so that the second bias current is applied to the dummy load, to prevent application of current transients to the second head. The sequence control then deasserts the dummy load to direct the second bias current to the second head.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.